The present invention concerns a transmitter circuit comprising a light source and arranged to operate said light source to transmit optical communication signals in response to electric input signals from a first and a second circuit point between which circuit points a balanced electric input signal is intended to be present. The invention also concerns a communication system comprising such a transmitter circuit.
Different transmitter circuits of the above mentioned kind are known. Such transmitter circuits are often used within telecommunication systems and data communication systems for transmitting optical signals in response to electric signals. The optical signals are usually transmitted in one or more optical fibres. The electric input signal is usually present as a balanced signal on a pair of electric conductors, for example on a twisted pair. Such a pair of electric conductors has a certain characteristic impedance, for example 100 ohm. In order to avoid undesired reflections, such a pair of electric conductors should in its end point be connected to a load which corresponds to the characteristic impedance.
It should be noted that by a balanced signal is meant that the signal that is present on the pair of conductors is such that the voltages on corresponding points on the two conductors are of the same magnitude but have opposite polarity relative to a reference potential. This reference potential is usually earth potential. With an unbalanced signal (or “single-ended”) is meant that the signal, i.e. the voltage variation, is only present on one conductor, while the other conductor, or the reference potential, is at a constant potential, usually on earth potential.
On a pair of conductors with a balanced signal, due to noise or other phenomena, a signal which is superposed on the two conductors may occur, a so-called common mode signal, which signal may vary with time. This signal is often undesired and should therefore be suppressed. This is often done with the help of, for example, transformers, baluns (a balun is a device which converts a balanced signal to an unbalanced signal) and differential amplifiers.
Also when a balanced electric signal is to be converted to an optical signal, such an undesired superposed signal need to be suppressed in order for the light source, which transmits the optical signal, to be correctly operated. According to the prior art, this has usually been done by first converting the balanced electric signal to an unbalanced electric signal.
FIG. 1 shows an example of the prior art. The electric balanced input signal is here present on a twisted pair 30. The balanced signal is converted to an unbalanced signal with the help of a balun 41 and a transformer 42. The circuit also comprises a termination resistance 43 which is adapted to the characteristic impedance of the twisted pair 30. Thereafter follows one or more circuits 44, which i.a. produce a suitable bias current and a modulation current, wherein the total current drives the light source 20.
Also EP-A-0 542 480 shows an example of a transmitter circuit. The transmitter circuit comprises two differentiators and an amplifier for driving a light emitting diode.
The prior known solutions are relative complicated and expensive, since they often comprise relatively complicated and expensive components, such as active components or transformers. Furthermore, known transmitter circuits often have a relatively high current consumption.
It should be noted that by active components is meant components which produce a gain or a switching, for example transistors, integrated circuits, and diodes.